User's Guide

SCSI Interface Description Page 15
Data Phase
The Data Phase is subdivided into the Data-In and Data-Out Phases.
Data-In Phase
During the Data-In phase, the Target requests that data be sent to the Initiator from
the Target. The Target asserts the I/O signal and de-asserts the C/D and MSG
signals thus denoting the Data-In Phase. The REQ/ACK handshakes (Figure 2-4)
then transfer the requested byte count.
Data-Out Phase
During a Data-Out phase, the Target requests that data be sent from the Initiator to
the Target. The Target de-asserts the C/D, I/O, and MSG signals thus denoting the
Data-Out Phase. The REQ/ACK handshakes (Figure 2-5) then transfer the
requested byte count across the SCSI bus.
Status Phase
During a Status Phase, the Target requests that status information be sent to the
Initiator from the Target. The Target asserts the C/D and I/O signals and de-asserts
the MSG signal thus denoting the Status Phase. The REQ/ACK handshakes the one
byte status code across the SCSI bus (refer to Figure 2-4).
Message Phase
The Message Phase consists of either the Message-In or Message-Out Phases.
The Seagate DDS-4 and DAT 72 drive supports one-byte messages.
Message-In Phase
During the Message-In Phase, the Target requests that messages be sent to the
Initiator from the Target. The Target asserts the C/D, I/O, MSG signals thus
denoting the Message-In Phase. The REQ/ACK handshakes the one-byte message
across the SCSI bus.
Message-Out Phase
During the Message-Out Phase, the Target requests that messages be sent from
the Initiator to the Target. The Target invokes this phase in response to the Attention
(ATTN) signal asserted by the Initiator. The drive responds to the ATTN signal at
every phase change. The Target asserts the C/D and MSG signals and de-asserts
the I/O signal, denoting the Message-Out Phase. The REQ/ACK handshakes the
one byte message across the SCSI bus. The Target uses REQ/ACK handshakes
(Figure 2-5) until the ATN signal becomes false, unless an error occurs and the
message is rejected.